Blast furnace stove control

ABSTRACT

A system for controlling the operation of a blast furnace stove during its on-gas period, including means for controlling the enrichment of the combustion fuel used to heat the stove. Control is effected primarily by comparing a measured stove dome temperature to a stove heating-up or firing schedule, represented by the output of a function generator programmed according to a desired stove temperature-time relationship, and referencing differences in the temperatures to a natural gas flow control loop which controls the richness of the fuel mixture and thereby increases or decreases the heat input to the stove, as required.

United States Patent [72] Inventor Donald W. ,Iohnson Pittsburgh, Pa. 2 1] Appl. No. 875,346 [22] Filed Nov. 10, 1969 [45] Patented Aug. 31, 1971 [73] Assignee Jones 8; Laughlin Steel Corporation Pittsburgh, Pa.

[54] BLAST FURNACE STOVE CONTROL 15 Claims, 1 Drawing Fig.

[52] U.S. Cl 263/19 A, 431/12, 431/76 511 1nt.Cl F23] 9/04 [50] Field of Search 431/12, 76; 263/19; 236/15 [56] References Cited UNITED STATES PATENTS 3,049,300 8/1962 Lewis et a1 431/12 X 3/1963 Krapf 3,241,597

3/1966 Juzi.....

- Primary ExaminerEdward G/Favors Attorneys-T A. Zalenski and'G. R. Harris ABSTRACT: A system for controlling the operation of a blast furnace stove during its on-gas period, including means for controlling the enrichmentof the combustion fuel used to heat the stove. Control is effected primarily by comparing a meato a natural gas flow control loop which controls the richness of the fuel mixture and thereby increases or. decreases the heat input to the stove, as required;

l I GAS "I' BURNER is :2 :7 0

l I L f 5 5f 55 56 FlRlN RATE 9 J "mam- TIME-LAG OMPENSATOR OXYGEN PU STATION BLAST FURNACE STOVE CONTROL This invention relates generally to blast furnace stoves and particularly to means for controlling the operation and enrichment of the combustion fuels of such stoves during their on-gas or firing cycle.

In the operation of a blast furnace, heated air is blown into the furnace at the bottom to burn the coke and any other fuel used. The heated air or blast is provided by passing relatively cold air through a blast furnace stove wherein the air is heated. A blast furnace stove is essentially a large heat sink composed of large quantities of refractory brickwork or checkers. initially, such a stove is operated on an on-gas or firing cycle, during which a gaseous fuel is burned at the stove burner located near the bottom of the stove and the hot combustion gages circulated through the stove and out the stove stack. After the checkerwork at the top or dome of the stove has reached some predetermined maximum temperature and the stack gases are at a temperature indicative of a certain quantity of heat having been stored in the checkerwork, firing is halted and the stove is placed on wind and cold air passed through the stove, heated therein and blown into the blast furnace. The cold air removes heat from the stove and, eventually, when the stove top checkerwork or dome reaches a predetermined minimum temperature, it is again put on a firing cycle.

Generally, the fuel used to heat the' stove is cleaned and scrubbed blast furnace top gas which is a byproduct of the blast furnace operation. This gas is mixed with combustion air and ignited in the stove at the stove burner. Blast furnace gas typically has a low B.t.u. content, ranging from about 70 to 95 B.t.u./cubic foot, depending on furnace operating conditions.

In modern blast furnace operation, a hot blast temperature of about 2,000 F. or above is desirable. The normal heating or firing cycle of a stove comprises a first period during which the stove dome temperature reaches a predetermined maximum, e.g., 2,300 F. and a second period during which the dome is held at that temperature to allow the stove checkerwork to absorb and store heat by soaking. To achieve and maintain a temperature of about 2,300 F. requires that the fuel ignited at the stove burner have an average B.t.u. content of about 85 B.t.u./cubic foot. Since blast furnace gas cannot be relied upon for such a heat content at all times, enrichment of the blast furnace gas, typically with natural gas, is necessary. The quantity of natural gas required to enrich the blast furnace gas is a function of the heat content of the blast furnace gas and it is desirable to provide a control scheme which will regulate the amount of natural gas used to achieve the desired on-gassoak-on-wind cycle time. This is necessary to achieve smooth furnace operation and keep natural gas purchases at a minimum.

Existing control schemes are based primarily on measuring the B.t.u. content of the blast furnace gas and adding a calculated amount of natural gas to bring the heat content of the gas burned in the stove to a theoretical level sufficient to heat the stove. B.t.u. analyzers are expensive however, and it is difficult to obtain the clean gas sample free of water required by the analyzers to make an accurate B.t.u. analysis. In addition, the feed forward function of B.t.u. analysis and calculated addition of enrichment assumes a limited deviation from the normal low temperature of the stove at the time it is placed on firing cycle, which is not always the case.

The control system of the present invention utilizes stove dome temperature to control stove operation during the firing cycle. Control is generally initiated some short time after stove firing is started, after sufficient time has expired to allow burner combustion to stabilize. Control is accomplished by comparing the measured dome temperature against a profile of desired dome temperature as a function of time as generated by a function generator. Any error is referenced to a natural gas flow control loop. If the measured dome temperature is greater than or equal to the reference profile, the

natural gas reference is negative and holds off natural gas.

flow. If the reference profile exceeds the dome temperature. a positive error is generated and natural gas flow is allowed. When the dome temperature has reached its predetermined maximum or set point (e.g. 2,300 F.), the total mass of stove checkerwork has only begun to absorb the heat it is capable of absorbing. However, further increase of the dome temperature is limited by the physical properties of the materials used in the stove construction. The control system will regulate the flow of natural gas so as to keep the dome temperature from exceeding its set point (e.g. 2.300 F.) until zero natural gas flow or shutoff.

When circumstances are such that the natural gas flow is zero and the measured dome temperature is at the predetermined maximum temperature, the blast furnace top gas will generally have more than enough of a heat content to hold the dome temperature at that maximum temperature. Therefore, the dome temperature will tend to exceed the predetermined maximum. To prevent this from occurring, excess air is added to the blast furnace gas to maintain the dome temperature at the desired level. The control operation continues until the stove stack temperature reaches a predetermined limit (approximately 750 F.) indicating that the checkers have absorbed as much heat as design allows and, consequently, that the stove is ready to be put on blast The system of the present invention also includes an oxygen loop. If too little air is mixed with the fuel for burning at the stove burner, a reducing atmosphere of unburned fuel is established in the stove causing checker deterioration and the likelihood of an explosion. If too much air is provided, an oxidizing atmosphere is established in the stove causing a reduction in the burner flame temperature and increasing stoveheating time. The oxygen loop is used to trim the combustion airflow according to the measurement of oxygen in the stove exhaust stack. This loop has an inherent time delay in it since air is mixed with the fuel at the burner while oxygen content is measured at the stack, and it may take as long as 30 seconds for changes made at the burner to be manifested at the stack,

depending upon stove design and mass flow rates.

An object of the present invention is to provide a system for controlling the operation of a blast furnace stove during its ongas or firing cycle. Another object is to control the enrichment of the gases burned during the on-gas cycle. Another object of the invention is to provide such control by controlling blast furnace gas flow, natural gas flow and airflow. Yet another object of the present invention is to achieve such control by providing a profile for desired stove dome temperature from the start of the firing cycle through achievement of dome temperature and through the time the dome is held at temperature while the checkers are reaching an even heat and comparing the profile temperature to the measured dome temperature. Still another object of the invention is to provide an oxygen analyzer loop to control airflow in response to the oxygen content of the stove exhaust gases, while compensating for transport delay.

These and otherobjects and advantages of the invention will become more apparent from the following description thereof with reference to the FlGURE of the drawing which schematically illustrates a blast furnace stove control system embodying the present invention. it will be understood that while the invention is particularly described herein with reference to instrumentation primarily utilizing electronic devices, known equivalent pneumatic, electrical and mechanical control devices can be employed.

The control system of the present invention regulates the blast furnace gas flow, the natural gas flow, and the airflow to the stove burner. Blast furnace gas flow regulation is achieved by a blast furnace gas flow control loop which continuously measures the blast furnace gas flow rate in the blast furnace gasline and controls that flow rate to substantially maintain the blast furnace gas flow to the stove burner at a preselected level. This control loop. includes first differential pressure to current transducer means 10 for sensing the differential pressure across the orifice plate 11 in the blast furnace gasline 12 leading to the stove burner and converting it to an electrical output signal of a magnitude depending on the magnitude of that differential pressure. This line provides the conduit through which cleaned and scrubbed blast furnace top gas passes to the burner.

To provide a signal which is a first power function of the blast furnace gas flow rate, the output of the transducer means 10, which because it is a function of the differential pressure in line 12 is a function of the blast furnace flow rate squared, is fed to first square root extractor means 13 which develops an output signal which is the square root of the output signal of the transducer means 10. Orifice plate 11, transducer means and square root extractor means 13 thus comprise means for measuring and providing an indication of the blast furnace gas flow rate in the blast furnace gasline 12.

Typically, a single blast furnace has associated with it two or more blast furnace stoves which are operated so that while one or more stoves are onwind, one or more stoves are ongas. From operating experience it is possible to determine generally the blast furnace gas flow which must be substantially maintained to a stove during its on-gas period so that at the time it is put on-wind, it is approximately at the requisite temperature. This preselected blast furnace gas flow level is incorporated into the control system of the invention as a reference level by firing rate set point means 14. The set point means comprises a linear potentiometer for developing an output signal measurably responsive to the preselected level of blast furnace gas flow in the same manner as the output of square root extractor means 13.

The outputs of set point means 14 and extractor 13 are fed to blast furnace gas loop controller means 15 which is a conventional proportional and reset controller. The controller means develops an output signal measurably responsive to the difference between the output signals of the set point means 14 and extractor means 13 and the time the difference has existed and is thus related both to the magnitude of the difference between the measured blast furnace flow rate and the preselected level of blast furnace gas flow to be established or if established to be maintained to the stove and the period of the time the difference has existed.

Means for controlling the action of a valve in the gasline 12 in response to the difference between the measured blast furnace gas flow rate and the blast furnace gas flow rate to be established or if established to be maintained comprises first current-to-pressure transducer means 16 which converts the signal developed by controllermeans 15 to a pneumatic pressure measurably responsive thereto. That pneumatic pressure acts to control the action of the valve 17 in the gas line 12 to regulate the blast furnace gas flow to the burner,

Blast furnace top gas, after it has been cleaned and scrubbed, typically has a composition of about 1.5 percent to 4.5 percent hydrogen, about 22 percent to 25 percent carbon monoxide, about 14 percent to 15 percent carbon dioxide, and remainder nitrogen. To render the gas combustible, oxygen, in the form of air, is mixed with it. A suitable mixture comprises about 1.4 parts blast furnace gas to one part air. As a first approximation in regulating airflow, the system of the invention operates on the basis of such a suitable fuel-to-air ratio. This is accomplished by means including fuel-to-air ratio means 18 for calling for a combustion airflow rate which is a preselected constant multiple of the measured blast furnace gas flow rate. Ratio means 18 has as its input the output of extractor means 13, representing the measured blast furnace flow rate. in response to that input, ratio means 18 develops an output signal which is representative of an airflow rate which is a constant multiple of the measured blast furnace gas flow rate, e.g., 0.7 of the blast furnace gas flow rate. This output is delivered to proportional and reset controller means 20.

The airflow rate is controlled not only in response to the blast furnace gas flow rate but also in response to the additional variables of the oxygen content of the gases issuing from the stove stack and the stove dome temperature. Changes in the oxygen content of the gases issuing from the stove stack affect airflow rate through an oxygen loop means and the stove dome temperature affects airflow rate through a means for calling for a greater combustion airflow rate than that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than a predetermined maximum temperature. The means for calling for a greater combustion airflow rate, the oxygen loop means and the ratio means collectively comprise means for calling for a combustion airflow rate to be established or if established to be maintained to the burner. The manner in which changes in airflow rate are made in response to changes in the oxygen content of the stove stack gases and the stove dome temperature is discussed in detail below. However, it is here noted that their influence is manifested as an electrical signal delivered to controller 20 through line 19.

A third input to controller 20 comprises the output of square root extractor means 21 which output is measurably responsive to the airflow rate in air line 24 leading to the stove burner. Extractor means 21 is operatively connected to differential pressure-to-current transducer means 22 and its output signal is the square root of the output signal of transducer 22. The output of transducer 22 is measurably responsive to the differential pressure which it senses across orifice plate 23 in air line 24. Orifice plate 23, transducer 22 and square root extractor means 21 thus comprise means for measuring and providing an indication of the combustion airflow in the air line 24.

In response to its three inputs, controller 20 develops a signal measurably responsive to the difference between the measured airflow rate and the airflow rate to be established or if established to be maintained to the stove and the length of time the difference has been existing. Means for controlling the action ofa valve in air line 24 in response to the difference between the measured combustion airflow rate and the combustion airflow rate to be established or if established to be maintained comprises current-to-pressure transducer means 25 which converts the signal developed by controller 20 to a pneumatic pressure measurably responsive thereto. That pressure acts to control the action of valve 26in air line 24 to regulate the airflow to the stove burner.

The system of the invention, as indicated above, controls stove heating during the on-gas period primarily by controlling natural gas flow to the burner. Natural gas flow is regulated by means of a natural gas flow control loop which functions in response to a temperature comparison circuit. The temperature comparison circuit develops an output which is measurably responsive to the difference between the instantaneous measured stove dome temperature and a desired temperature, as indicated by a stove dome temperature-time schedule representing a desired heating-up program for the stove. The circuit comprises temperature profile function generator 30, dome temperature sensor 31 and proportional controller means 32.

The basic reference for the control system is temperature profile function generator means 30 which provides a representation of the temperature desired to be established in the stove dome as a function of time during the on-gas cycle. The output of function generator 30 is measurably responsive to the temperature at which it is desired the stove dome temperature be at any instant during the on-gas period. In heating up or firing a blast furnace stove, the normal practice is to heat the stove so that the stove dome temperature rapidly reaches a desired maximum and is maintained there while the checkers continue to soak up heat until they have absorbed a quantity .of heat according to their design. A desired heating-up schedule can be arrived at from a consideration of optimum past practices and the function generator programmed in accordance therewith so that its output with time represents a temperature profile which tracks the desired heating-up schedule.

The stove dome temperature is measured by a temperaturesensing means 31, such as a Ray-O-Tube, manufactured by Leeds and Northrup, mounted in the stove dome. The sensor develops an output signal measurably responsive to the temperature of the the top checkerwork and that signal along with the output signal of the function generator 30 is delivered to natural gas flow proportional controller means 32. The controller means 32 in response thereto develops an output signal measurably responsive to the difference between the measured stove dome temperature and the desired stove dome temperature. This signal is related to some natural gas flow rate, including zero, which should be established or if established should be maintained to the stove burner to effectuate or maintain the desired stove temperature.

The output of controller 32 is delivered to the natural gas flow control loop. There, it sets natural gas flow set point means 33 which keeps the natural gas flow rate called for by controller means 32 from exceeding a predetermined maximum which is based on the blast furnace gas flow rate desired to be maintained to the burner. Set point means 33 comprises a linear potentiometer for developing up to a preset maximum a signal measurably responsive to the output of controller 32. The preset maximum is a function of the preselected level of blast furnace gas flow substantially maintained to the burner as set by firing rate set point means 14. Instances may occur where the measured dome temperature is so far below the desired dome temperature that the natural gas flow rate needed to rapidly bring the dome temperature to the desired level according to the programmed reference would be so great with relation to the level of blast furnace gas flow set in set point means 14 that the richness of the fuel delivered to the burner would result in excessive burner flame temperatures, resulting in accelerated destruction or weakening of the materials used in the construction of the stove. In this regard, it is preferred that the natural gas flow rate be no greater than about 2 percent of the blast furnace gas flow rate. Accordingly, firing rate set point means 14 and natural gas set point means 33 are appropriately interconnected so that when the firing rate set point means is set, the natural gas set point means is automatically set at a maximum representing a natural gas flow rate about 2 percent of the blast furnace gas-firing rate; in this manner, irrespective of the measured dome temperature, the output of set point means will not exceed the preset maximum. For example, by setting set point means 14 to represent a blast furnace firing rate of 30,000 cubic feet per minute (c.f.m.), the natural gas set point means 33 is automatically set at a maximum representing a natural gas flow rate of 600 c.f.m.; and regardless of the difference between the measured dome temperature and the desired stove temperature, the output of controller means 32, which is a measure of the temperature difference, is unable to drive set point means 33 beyond the point representing a natural gas flow rate of 600 c.f.m.

it can thus be understood that controller 32 and set point means 33 function as means for developing up to a preset maximum and in response to the measured stove temperature and the desired stove temperature, as represented by the output of function generator 30, an output signal which is measurably responsive to an increased natural gas flow rate when the desired stove temperature exceeds the measured stove temperature and a decreased natural gas flow rate when the measured stove temperature exceeds the desired stove temperature.

it is also possible that, from time to time, the blast furnace top gas flow would become so low that the firing rate set in means 14 could not be maintained. At the same time, the natural gas flow called for by set point means 33, while it might be less than about 2 percent of the firing rate set in means 14, could be greater than about 2 percent of the reduced blast furnace gas flow. Thus, considering the example above, the blast furnace gas flow rate would fall below 30,000 c.f.m. to say, for example, 20,000 c.f.m. despite the fact that valve 17. would be full open. At the same time the natural gas flow called for bycontroller 32 could be in excess of 400 c.f.m. (2 percent of 20,000 c.f.m.) although below 600 c.f.m. To provide for this contingency, soas to minimize the likelihood of excessive flame temperatures and weakening of the material of stove construction, the output of set point means 33 is delivered to modifier means 34 along with the output of extractor means 13. The output of the extractor 13 is scaled to the same basis as the output of set point means 33 and the modifier means compares these two signals, determines which signal is smaller and applies that signal to conventional proportional and reset controller means 35. Modifier 34 thus keeps the natural gas flow rate called for below a preselected percentage of the measured blast furnace gas flow rate to the burner. In this manner the system allows for natural gas enrichment as a function of actual blast furnace gas flow rates rather than the set point rate, if the actual blast furnace gas flow rate is below the blast furnace gas set point.

The natural gas flow control loop further includes differential pressure to current transducer means 36 for sensing the differential pressure across the orifice plate 37 in the natural gas line 38 leading to blast furnace gasline 12 and converting the pressure to an electrical output signal of a magnitude dependent thereon. Second square root extractor means 39 is operatively connected to transducer means 36 for developing an output signal which is the square root of the output signal of transducer means 36. Thus, orifice plate 37, transducer means 36and extractor means 39 comprise means for measuring and providing an indication of the natural gas flow rate in the natural gas line.

The output of extractor means 39 is fed to proportional and reset controller means 35 along with the output of modifier 34, and the controller in response thereto develops a signal measurably responsive to the difference between those outputs and the time the difference has existed. This signal is thus related both to the magnitude of the difference between the measured natural gas flow rate and the natural gas flow rate to be established or if established to be maintained to the stove and the period of time the difference has existed.

Means for controlling the action of a valve in natural gas line 38 in response to the difference between the measured natural gas flow rate and the natural gas flow rate to be established or if established to be maintained comprises current to pressure transducer means 40 which converts the signal developed by controller means 35 to a pneumatic pressure measurably responsive thereto. That pressure acts to control the action of valve 41 in natural gas line 38 to regulate the natural gas flow to blast furnace gasline 12, the two gases mixing at intersection 42.

The output of function generator 30 is not activated nor is natural gas enrichment initiated until the closing of relay 43 which occurs some little time after stove firing is initiated. Relay 43 both resets and activates function generator 30. It first resets the generator to time scale zero at the completion of the on-wind cycle of the stove and initialization of stove burner combustion. A measured time later, e. g., 30 seconds, it activates the generator.

In addition to controlling blast furnace and natural gas flows, the system of the invention controls airflow to the stove burner by continuously measuring the airflow rate in the air line and controlling that flow rate so as to provide both an oxygen level at the burner in excess of that required to oxidize all the combustible gases thereat and a sufficient airflow to lower the stove temperature when the natural gas flow is zero and the measured stove temperature exceeds a preselected maximum temperature. Airflow to the stove burner is thus controlled in response to three variables; the blast furnace gas flow rate, the oxygen content of the gases issuing from the stove stack and the stove dome temperature. As described above, the basic variable on which the airflow control functions is in the blast furnace gas rate. The manner in which the other two variables influence airflow will now be described.

The ratio of air to fuel delivered to the burner is controlled primarily by controlling the flow of air as a proportion of fuel flow pipe on each side of the orifice plate. This length of pipe is completely impracticable for a blast furnace stove because it requires too great a length of unimpeded pipe for a typical blast furnace installation. Consequently, the orifice plates will not provide precisely accurate readings over a large range of flows and in many instances too little air (reducing atmosphere) or too much air (oxidizing atmosphere) is present in the stove during firing. A reducing atmosphere causes checker deterioration and creates the likelihood of an explosion due to incomplete combustion. Too much air reduces the flame temperature at the burner and lengthens the heating cycle. Accordingly, the present system trims combustion airflow according to the oxygen content of the'stove stack gases. This is accomplished by an oxygen loop means for calling for an increase in the combustion airflow rate over that called for by ratio means 18 when the oxygen gas content of the gases issuing from the stove exhaust stack is below a preselected level and a decrease in the combustion airflow rate below that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is above said preselected level. To measure the oxygen content of the stack gases, a stack gas sample line 46 is inserted in stove stack 47 whereby samples of the gas passing through the stack pass into line 46. The gas samples then pass through a sample clean and pump station 48 where moisture and dirt particles are removed and from there the gas samples pass onto the oxygen analyzer 49. The oxygen analyzer determines the oxygen content of the gas samples and develops a signal measurably responsive to that oxygen content. That output is then fed to the conventional combustion airflow proportional controller means 50.

Operating experience shows that an airflow which is about l percent or percent in excess of the air required for complete combustion of all the fuel, both natural gas and blast furnace gas, delivered to the stove burner should be maintained. Oxygen set point means 51, which comprise a linear potentiometer, is provided in the oxygen loop means for selecting a range of excess air percentage which is to be maintained, the output of the set point means 51 being measurably responsive to the excess air value selected. This output is fed to summing amplifier 52 along with the output of controller means 61, in those instances where an an output is present at controller 61, and the sum of those two outputs delivered to controller 50. The output of controller means 61 represents the influence of the dome temperature on airflow as is more fully described below. Controller 50 develops an output signal measurably responsive to the difference between the signal developed by summing means 52 and the signal developed by oxygen analyzer means 49 thereby calling for an increase in the combustion airflow rate over that called for by ratio means 18 when the oxygen gas content of the gases issuing from the stove stack is below the preselected level of oxygen set point 51 and a decrease in the combustion airflow rate below that called for by ratio means 18 when the oxygen gas content of those gases is above that preselected level. The output of controller 50 is then delivered to time lag compensator means 53. This occurs only after closing of relay 54 which is controlled by a timer set to activate the relay when burner combustion has stabilized.

It will be appreciated that the system of the present invention has an inherent time delay since the air is introduced at the burner while oxygen content is measured adjacent the stack and it may take as long as 30 seconds for a change at the burner to be manifested by a change at the stack and analyzer 49. This time delay is referred to as transport time and if not properly compensated for can cause system instability. The system of the present invention compensates for transport time by providing in the oxygen loop a sampling circuit in the form of time lag compensator means 53 which keeps the mag nitude of the increase or decrease in airflow rate called for out of controller 50 constant for intermittent time periods in order to compensate for the time which is required for changes made in the airflow rate to be manifested at analyzer 49. Time lag compensator 53 examines the output of controller 50 for a short period of time referred to as on-time, this output being integrated at the compensator during that period at a slow rate. At the end of the on-time integrated signal is held constant for a much longer period of time referred to as off-time. The length of the off-time period is selected to be long enough to allow changes made in the air-fuel mixture to be manifested at the oxygen analyzer. At the end of the off-time the on-time is again initiated. One of three events may then occur: The integrated signal will increase in response to a positive signal from controller 50, indicating an oxygen analysis lower than that called for by summing amplifier 52. The integrated signal will decrease in response to a negative signal from controller 50, indicating an oxygen analysis higher than that called for by summing amplifier S2. The integrated signal will remain constant in response to a zero signal from controller 50, indicating an oxygen analysis the same as that called for by summing amplifier 52. At the end of the new on-time, a new off-time begins with the signal developed by the compensator 53 at the end of that new on-time being maintained during the new offtime. This procedure continues throughout the on-time period. Suitable onand offtimes are l and 10 seconds respectively, with the off-time being adjusted as a function of blast furnace gas rate.

Considering now the influence now the influence of the dome temperature on the airflow rate, as described above, when the measured stove temperature falls below the desired temperature, as represented by the output of the function generator 30, the natural gas flow rate is increased, and when the measured stove temperature is greater than the desired temperature, the natural gas flow rate is decreased. However, there are instances in the heating up of the checkerwork of a stove when the measured dome temperature is greater than the desired temperature but the natural gas flow rate is zero so that temperature control cannot be accomplished by natural gas control. To provide for this contingency, the system of the present invention, in such circumstances, acts to increase the airflow to the stove burner and the increased airflow acts to cool the combustion flame. This is accomplished by means for calling for an increase in the combustion airflow rate over that called for by ratio means 18 and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than a predetermined-maximum temperature. By delivering the output of dome temperature set point means 60 and the output of dome temperature sensor means 31 to proportional controller means 61, the latter in response to these two signals develops an output signal related to the mag nitude of their difference. Controller means 61 is activated only by the closing of relay 62 which occurs when the natural gas flow rate becomes zero. Dome temperature set point means 60 comprises a linear potentiometer which develops a signal measurably responsive to a dome temperature set thereon. The set point means 60 is set at the maximum temperature at which it is desired the stove dome attain. Delivery of the output of set point means 60 to controller 61 does not occur until the closing of relay 63 which occurs at the same time relay 54 closes.

The airflow is controlled in one of two ways. According to a first method, the airflow rate to the burner is based on the fuel-to-air ratio to be maintained, as established by ratio means 18, modified by additions of air as called for by con troller 61 whenever the natural gas flow is zero and the measured stove temperature exceeds the profile temperature. According to a second method, the airflow rate is additionally modified on the basis of the oxygen content of the stove stack gases. Selection of the method of operation is accomplished by means of relays 55 and 56 and OR gate 57. When operation 13. The system of claim 12 including time-lag compensator means for alternately integrating the electrical error signal developed by the combustion airflow proportional controller means during a first period of time while developing an electrical output signal during that period of time as a function of the integration and maintaining the integrated signal developed at the end of the first period of time constant for a following second period of time whereby to allow for changes made in the combustion airflow rate to be manifested at the oxygen analyzer means.

14. The system of claim 13 including natural gas set point means for developing an electrical signal in response to the electrical signal developed by the natural gas flow proportional controller means and for keeping the electrical signal which it develops from exceeding a predetermined maximum which is representative of a predetermined maximum natural gas flow rate based on the blast furnace gas flow rate desired to be maintained to the burner.

15. The system of claim 14 including modifier means for developing an electrical signal in response to the electrical signal developed by the natural gas set point means and for keeping the electrical signal which it develops below a value representative of a maximum preselected percentage of the measured blast furnace gas flow rate to the burner.

according to the first method is desired, relay 56 is closed while relay 55 is opened and the output of controller 61 is delivered to controller 20. Operation according to the second method occurs by closing relay 55 and opening relay 56 so that the output of compensator 53 is delivered to controller 20.

lCLAlM:

l. A system for controlling the operation of a blast furnace stove during its on-gas period, said stove having a stove burner and associated blast furnace gas, natural gas and combustion air lines for conveying blast furnace gas, natural gas and combustion air respectively to the stove burner, said lines each including both means for measuring and providing an indication of the gas flow rate therein and means for controlling the action of a valve in the line in response to the difierence between the measured gas flow rate and a gas flow rate to be established or if established to be maintained in the line including a. means for providing a representation'of the temperature desired to be established in the stove dome as a function of time during the on-gas cycle,

b. means connected to means (a) for comparing the actual stove dome temperature and the desired stove dome temperature and calling for a natural gas flow rate to be established or if established to be maintained to the burner as a function of the magnitude of the difference between the temperatures and c. means for calling for a combustion airflow rate to be established or if established to be maintained to the burner including ratio means for calling for a combustion airflow rate which is a preselected constant multiple of the measured blast furnace gas flow rate.

2. The system of claim 1 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes oxygen loop means for calling for an increase in the combustion airflow rate over that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is below a preselected level and a decrease in the combustion airflow rate below that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is above said preselected level.

3. The system of claim 2 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes means calling for an increase in the combustion airflow rate over that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than a predetermined maximum temperature.

4. The system of claim 2 wherein the oxygen loop means includes means for keeping the magnitude of the increase or decrease in the airflow rate called for constant for intermittent time periods in order to compensate for the time which is required for changes made in the airflow rate to be manifested at a means for measuring the oxygen content of the gases issuing from the stove stack.

5. The system of claim 1 including means for keeping the natural gas flow rate called for from exceeding a predetermined maximum which is based on the blast furnace gas flow rate desired to be maintained to the burner.

6. The system of claim 5 including means for keeping the natural gas flow rate called for below a preselected percentage of the measured blast furnace gas flow rate to the burner.

7. The system of claim 6 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes oxygen loop means for calling for a greater airflow rate than called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is below a preselected level and a lesser airflow rate than that called for by the ratio means when the oxygen gas content of the gas issuing from the stove exhaust stack is above said selected level.

8. The system of claim 7 wherein the means for calling for a combustion airflow rate tobe established or if established to be maintained to the burner further includes means calling for a greater airflow rate than that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than a predetermined maximum temperature.

9. The system of claim 7 wherein the oxygen loop means includes means for keeping the greater or lesser flow rate called for constant for intermittent time periods in order to compensate for the time which is required for changes made in the airflow rate to be manifested at a means for measuring the oxygen content of the gases issuing from the stove stack.

10. A system for controlling the operation of a blast furnace stove during its ongas period, said stove having a stove burner and associated blast furnace gas, natural gas and combustion air respectively to the stove burner, said lines each including both means for measuring and providing an indication of the gas flow rate therein and means for controlling the action of a valve in the line in response to the difference between the measured gas flow rate and a gas flow rate to be established or if established to be maintained in the line including a. temperature profile function generator means for providing an electrical output representative of the temperature desired to be established in thestove dome as a function of time during the on-gas cycle,

b. sensing means for measuring the stove dome temperature and developing an electrical signal measurably responsive thereto,

c. natural gas flow proportional controller means for developing an output signal measurably responsive to the difference between the signals of the temperature profile function generator means and the sensing means, said signal being representative of a natural gas flow rate to be established or if established to be maintained to the burner as a function of the magnitude of the difference between the desired stove dome temperature and actual stove dome temperature,

d. means for developing an electrical signal representative of a combustion airflow rate to be established or if established to be maintained to the burner including ratio means for developing an electrical signal representative of a combustion airflow rate which is a preselected constant multiple of the measured blast furnace gas flow rate.

11. The system of claim 10 wherein the means for developing an electrical signal representative of a combustion airflow rate to be established or if established to be maintained to the burner further includes oxygen analyzer means for developing an electrical signal representative of the oxygen gas content of the gases issuing from the stove stack and a combustion airflow proportional controller means for developing, in response to the electrical signal developed by the oxygen analyzer means and an electrical signal representative of an oxygen gas content desired to be maintained in the exhaust gases issuing from the stove stack, an electrical signal representative of an increase in the combustion airflow rate over that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is below the desired level and a decrease in the combustion airflow rate below that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is above said desired level.

12. The system of claim 11 wherein the means for developing an electrical signal representative of a combustion airflow rate to be established or if established to be maintained to the burner further includes proportional controller means for developing, in response to the electrical signal developed by the temperature-sensing means and an electrical signal representative of a preselected maximum dome temperature, an electrical signal representative of an increase in the combustion airflow rate over that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than the preselected maximum dome temperature. 

1. A system for controlling the operation of a blast fuRnace stove during its on-gas period, said stove having a stove burner and associated blast furnace gas, natural gas and combustion air lines for conveying blast furnace gas, natural gas and combustion air respectively to the stove burner, said lines each including both means for measuring and providing an indication of the gas flow rate therein and means for controlling the action of a valve in the line in response to the difference between the measured gas flow rate and a gas flow rate to be established or if established to be maintained in the line including a. means for providing a representation of the temperature desired to be established in the stove dome as a function of time during the on-gas cycle, b. means connected to means (a) for comparing the actual stove dome temperature and the desired stove dome temperature and calling for a natural gas flow rate to be established or if established to be maintained to the burner as a function of the magnitude of the difference between the temperatures and c. means for calling for a combustion airflow rate to be established or if established to be maintained to the burner including ratio means for calling for a combustion airflow rate which is a preselected constant multiple of the measured blast furnace gas flow rate.
 2. The system of claim 1 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes oxygen loop means for calling for an increase in the combustion airflow rate over that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is below a preselected level and a decrease in the combustion airflow rate below that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is above said preselected level.
 3. The system of claim 2 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes means calling for an increase in the combustion airflow rate over that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than a predetermined maximum temperature.
 4. The system of claim 2 wherein the oxygen loop means includes means for keeping the magnitude of the increase or decrease in the airflow rate called for constant for intermittent time periods in order to compensate for the time which is required for changes made in the airflow rate to be manifested at a means for measuring the oxygen content of the gases issuing from the stove stack.
 5. The system of claim 1 including means for keeping the natural gas flow rate called for from exceeding a predetermined maximum which is based on the blast furnace gas flow rate desired to be maintained to the burner.
 6. The system of claim 5 including means for keeping the natural gas flow rate called for below a preselected percentage of the measured blast furnace gas flow rate to the burner.
 7. The system of claim 6 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes oxygen loop means for calling for a greater airflow rate than called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is below a preselected level and a lesser airflow rate than that called for by the ratio means when the oxygen gas content of the gas issuing from the stove exhaust stack is above said selected level.
 8. The system of claim 7 wherein the means for calling for a combustion airflow rate to be established or if established to be maintained to the burner further includes means calling for a greater airflow rate than that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than a predetermined maximum temperaturE.
 9. The system of claim 7 wherein the oxygen loop means includes means for keeping the greater or lesser flow rate called for constant for intermittent time periods in order to compensate for the time which is required for changes made in the airflow rate to be manifested at a means for measuring the oxygen content of the gases issuing from the stove stack.
 10. A system for controlling the operation of a blast furnace stove during its on-gas period, said stove having a stove burner and associated blast furnace gas, natural gas and combustion air respectively to the stove burner, said lines each including both means for measuring and providing an indication of the gas flow rate therein and means for controlling the action of a valve in the line in response to the difference between the measured gas flow rate and a gas flow rate to be established or if established to be maintained in the line including a. temperature profile function generator means for providing an electrical output representative of the temperature desired to be established in the stove dome as a function of time during the on-gas cycle, b. sensing means for measuring the stove dome temperature and developing an electrical signal measurably responsive thereto, c. natural gas flow proportional controller means for developing an output signal measurably responsive to the difference between the signals of the temperature profile function generator means and the sensing means, said signal being representative of a natural gas flow rate to be established or if established to be maintained to the burner as a function of the magnitude of the difference between the desired stove dome temperature and actual stove dome temperature, d. means for developing an electrical signal representative of a combustion airflow rate to be established or if established to be maintained to the burner including ratio means for developing an electrical signal representative of a combustion airflow rate which is a preselected constant multiple of the measured blast furnace gas flow rate.
 11. The system of claim 10 wherein the means for developing an electrical signal representative of a combustion airflow rate to be established or if established to be maintained to the burner further includes oxygen analyzer means for developing an electrical signal representative of the oxygen gas content of the gases issuing from the stove stack and a combustion airflow proportional controller means for developing, in response to the electrical signal developed by the oxygen analyzer means and an electrical signal representative of an oxygen gas content desired to be maintained in the exhaust gases issuing from the stove stack, an electrical signal representative of an increase in the combustion airflow rate over that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is below the desired level and a decrease in the combustion airflow rate below that called for by the ratio means when the oxygen gas content of the gases issuing from the stove exhaust stack is above said desired level.
 12. The system of claim 11 wherein the means for developing an electrical signal representative of a combustion airflow rate to be established or if established to be maintained to the burner further includes proportional controller means for developing, in response to the electrical signal developed by the temperature-sensing means and an electrical signal representative of a preselected maximum dome temperature, an electrical signal representative of an increase in the combustion airflow rate over that called for by the ratio means and the oxygen loop means when the natural gas flow is zero and the measured dome temperature is greater than the preselected maximum dome temperature.
 13. The system of claim 12 including time-lag compensator means for alternately integrating the electrical error signal developed by the combustion airflow proportional controller means during a firSt period of time while developing an electrical output signal during that period of time as a function of the integration and maintaining the integrated signal developed at the end of the first period of time constant for a following second period of time whereby to allow for changes made in the combustion airflow rate to be manifested at the oxygen analyzer means.
 14. The system of claim 13 including natural gas set point means for developing an electrical signal in response to the electrical signal developed by the natural gas flow proportional controller means and for keeping the electrical signal which it develops from exceeding a predetermined maximum which is representative of a predetermined maximum natural gas flow rate based on the blast furnace gas flow rate desired to be maintained to the burner.
 15. The system of claim 14 including modifier means for developing an electrical signal in response to the electrical signal developed by the natural gas set point means and for keeping the electrical signal which it develops below a value representative of a maximum preselected percentage of the measured blast furnace gas flow rate to the burner. 